Powder metal rotating components for turbine engines and process therefor

ABSTRACT

A process for producing turbine rotors and other large rotating components of power-generating gas turbine engines using powder metallurgy techniques. The process involves forming a powder of a gamma prime or gamma double prime precipitation-strengthened nickel-based superalloy whose particles are about 0.100 mm in diameter or smaller. The powder is placed in a can and consolidated to produce an essentially fully dense consolidation, which is then hot worked to produce a billet of a size sufficient to form a forging of at least 2300 kg. The billet is forged at a temperature and strain rate to produce a forging with a uniform fine grain of ASTM 10 or finer. Thereafter, the forging may undergo a heat treatment to achieve a desired balance of mechanical properties while retaining a uniform grain size of ASTM 10 or finer.

BACKGROUND OF THE INVENTION

This invention relates to processes for producing large forgings usingmetal powders as the starting material. More particularly, thisinvention is directed to a process for producing turbine rotors andother large rotating components of turbine engines using powdermetallurgy techniques.

Rotor components for certain advanced land-based gas turbine enginesused in the power-generating industry, such as the H and FB class gasturbines of the assignee of this invention, are currently formed fromgamma double-prime (γ″) precipitation-strengthened nickel-basedsuperalloys, such as Alloy 718 and Alloy 706. For example, wheels andspacers have been formed from triple-melted (vacuum induction melting(VIM)/electroslag remelting (ESR)/vacuum arc remelting (VAR)) ingotswith diameters of about 27 to 36 inches (about 70 to about 90 cm), whichare then billetized and forged. Due to potential chemical ormicrostructural segregation and anticipated hot working losses goingfrom ingot to final forging, starting ingot weights must be from about1.5 to 3 times the weight of the finished forging, and about 2.5 to 7times the weight of the finish-machined part. In addition to thesesubstantial material losses, the best current processing practicestypically result in nonuniform and relatively coarse-grainedmicrostructures in the billet (e.g., ASTM 00 or larger) and the finishforgings (e.g., ASTM 8.0 or larger) (reference throughout to ASTM grainsizes is in accordance with the standard scale established by theAmerican Society for Testing and Materials). The billet grain size istoo large to permit any adequate ultrasonic inspection to identifypotential life limiting defects and is consequently not performed oncurrently used billet. The finished forgings must therefore beultrasonically inspected for potential life-limiting defects, andtypically necessitate a minimum 0.25 inch (about 6 mm) thick sonic shapeinspection envelope that defines the finished forged shape envelope.

In contrast, rotor components for aircraft gas turbine engines haveoften been formed by powder metallurgy (PM) processes, which are knownto provide a good balance of creep, tensile and fatigue crack growthproperties to meet the performance requirements of aircraft gas turbineengines. Typically, a powder metal component is produced byconsolidating metal powders in some form, such as extrusionconsolidation, then isothermally or hot die forging the consolidatedmaterial to the desired outline, and finally heat treating the forgingbefore finish machining to complete the manufacturing process. Theprocessing steps of consolidation and forging are designed to retain avery fine grain size within the material to enable high resolutionultrasonic inspection of billets, minimize die loading, and improveshape definition of the finished forging. Unlike advanced turbinesystems for land-based gas turbine engines, PM rotor components foraircraft gas turbine engines have been typically formed from gamma prime(γ′) precipitation-strengthened nickel-based superalloys with very hightemperature and stress capabilities demanded by those parts. In order toimprove the fatigue crack growth resistance and mechanical properties atelevated temperatures, some of these alloys are heat treated above theirgamma prime solvus temperature (generally referred to as supersolvusheat treatment) to cause significant, uniform coarsening of the grains.The nickel-based superalloy rotors used in large electrical powergenerating turbines currently do not require the higher temperaturegamma prime alloys nor this grain coarsening process to meet theirmission and component mechanical property requirements, though it isforeseeable that such higher temperature alloys could be required atsome future date to increase turbine efficiencies or increase componentlife.

While powder metal nickel-based superalloys have been processed for usein aircraft engine turbine rotor forgings, whose forgings are typicallyless than 2000 pounds (about 900 kg), powder metal techniques have notbeen used to produce the significantly larger forgings required by gasturbines used in the power-generating industry, which can weigh inexcess of 5000 pounds (about 2300 kg). However, the ability to use apowder metallurgy process to produce large nickel-based superalloyforgings suitable for rotor components of power-generating gas turbineengines would provide the capability of producing more near-net-shapeforgings, thereby reducing material losses. Until recently, these powergeneration turbine alloys were iron or nickel-based with low alloycontent, i.e., three or four primary elements, which permit theirmelting and processing with relative ease and minimal chemical ormicrostructural segregation. Powder metal versions of these alloys wouldoffer no significant benefit, either in ease of processing or propertygains, to compensate for the higher base cost of PM compared to the castingots which can be readily converted into rotor forgings. However, asmore complex alloys such as Alloy 718 and beyond become preferred andthe size of forgings continues to increase, the concerns of chemical andmicrostructure segregation, high material losses associated withconverting large grained ingots to finish forgings, and limited industrycapacity to process large, high strength forgings make the higher basecost PM alloys potentially more cost effective. Reduced processinglosses, expanded industry capacity, improved inspectibility of finegrain PM billets and parts, and the ability to produce morenear-net-shape forgings are all contributing factors to achieving lowercost large rotor forgings from PM than from the current cast pluswrought practice.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a process for producing turbine rotorsand other large rotating components of power-generating gas turbineengines using powder metallurgy techniques. The method significantlyreduces the ratio of input weight to final forging weight by eliminatingyield losses during conversion from large grained ingot to a finegrained forging. The method also virtually eliminates chemical andmicrostructural segregation, and results in a fine, uniform grain size(ASTM 10 or finer) that advantageously reduces the required sonic shapeenvelope and therefore further reduces the finish forging weight.Additionally, the use of fine grain PM billet has the capability ofreducing the press forces required to produce finish forgings, therebyreducing capital equipment cost and expanding the potential supplierbase.

The process of this invention involves forming a powder of aprecipitation-strengthened (gamma prime or gamma double prime)nickel-base superalloy whose particles are about 0.004 inch (about 0.100mm) in diameter or smaller. The powder is placed in a can, which isevacuated and sealed in a controlled environment and then consolidatedat a temperature, time, and pressure to produce an essentiallyfully-dense consolidation. The consolidation is then hot worked at atemperature to produce a billet with a uniform grain size of ASTM 10 orfiner and of a size sufficient to form a forging of at least 5000 pounds(about 2300 kg). The billet is then forged at a temperature and strainrate selected to produce a forging with a uniform fine grain of ASTM 10or finer throughout. Thereafter, the forging preferably undergoes a heattreatment designed to achieve a desired balance of mechanical propertieswhile retaining a grain size of ASTM 10 or finer.

As a result of the above process, very large rotor components that werepreviously limited to processing by conventional cast and wroughttechniques may now be formed by powder metallurgy techniques withreduced material losses, as well as microstructural, compositional, andmechanical property advantages that can be achieved with powdermetallurgy processes.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a process for manufacturing very largenickel-base alloy rotor forgings, generally in excess of 5000 pounds(about 2300 kg), using powder metallurgy techniques. Powder metal alloysare used to produce nickel-base consolidations, which are then hotworked into billets and subsequently forged into large turbine wheels,spacers, or other rotating components of a size suitable for large gasturbine engines used in the power generating industry.

A particularly suitable alloy for illustrating the advantages of thisinvention is a gamma-prime precipitation-strengthened nickel-basesuperalloy based on the commercially-available Alloy 725. Thesuperalloy, identified herein as ARA725, has a composition of, byweight, about 19 to about 23% chromium, about 7 to about 8% molybdenum,about 3 to about 4% niobium, about 4 to about 6% iron, about 0.3 toabout 0.6% aluminum, about 1 to about 1.8% titanium, about 0.002 toabout 0.004% boron, about 0.35% maximum manganese, about 0.2% maximumsilicon, about 0.03% maximum carbon, the balance nickel and incidentalimpurities. Properties of conventionally cast plus wrought ARA725 citedin U.S. Pat. No. 6,315,846 to Hibner et al. and U.S. Pat. No. 6,531,002to Henry et al. that are believed to render the alloy particularly wellsuited for producing very large forgings from powder metal include roomand elevated temperature tensile strength and ductility similar to Alloy718 with significantly improved time dependent crack growth resistancecompared to Alloy 718. Though no mechanical property data is yetavailable for a powder metallurgy version of ARA725, it is anticipatedthat a properly processed powder metal forging will have similar orpossibly better properties than the cast plus wrought forgings. Whilethe invention will be described in reference to the ARA725 alloy, theteachings of this invention are applicable to other gamma prime andgamma double prime precipitation-strengthened nickel-based superalloys,such as Alloy 625, LC Astroloy (U700), Udimet 720, ARA054, ARA017, andany other nickel-based superalloy with tensile properties equal to orbetter than Alloy 718 combined with superior time dependent crack growthresistance compared to Alloy 718.

For the applications of interest to the invention, optimumprocessibility and mechanical properties are achieved by uniform grainsizes of not larger than ASTM 10. Grain sizes larger than ASTM 10 areundesirable in that the presence of such grains can significantly reducethe low cycle fatigue resistance of the component, can have a negativeimpact on other mechanical properties of the component such as tensileand high cycle fatigue (HCF) strength, increase hot working loadrequirements, and inhibit the thorough ultrasonic inspection of billetsand thick section forgings. Therefore, a preferred aspect of thisinvention is to achieve a uniform grain size within a nickel-basesuperalloy, in which random grain growth is prevented so as to yield amaximum grain size of ASTM 10 or finer.

The process of this invention involves forming a melt whose chemistry isthat of the desired alloy (e.g., ARA725). This is typically accomplishedby VIM processing but could also be performed by adaptation of ESR orVAR processes to provide melt for subsequent atomization or other powdermaking method. In view of the reactivity of elements (e.g., aluminum andtitanium) contained in preferred gamma prime and gamma double primeprecipitation-strengthened alloys, the melt is formed under vacuum or inan inert environment (hereinafter, a controlled environment). While inthe molten condition and within chemistry specifications, the alloy isconverted into powder by atomization or another suitable process toproduce generally spherical powder particles. According to a preferredaspect of the invention, the particles are produced by atomization tohave diameters of predominantly 0.004 inch (about 0.100 mm) or smaller.The powder is then sieved in a controlled environment to removeessentially all particles larger than 0.004 inch (about 0.100 mm) forthe purpose of reducing the potential for defects in the subsequentbillet/forgings. Larger powder sizes may be acceptable if defectparticles (e.g., ceramics, etc.) larger than 0.004 inch (about 0.100 mm)can be removed other than by a screening process. Because of the largequantity of powder required to produce billets of the size required bythis invention, e.g., 5000 to 20,000 pounds (about 2300 to about 10,000kg), it may be necessary to blend powders produced from multipleatomization steps to accumulate sufficient powder for use in the processof this invention. Any required storage of such powders is preferably ina controlled environment container.

Once a sufficient amount of powder has been produced, the powder isplaced in a suitable can, preferably a mild steel can, whose size willmeet the billet size requirement after consolidation. Loading of the canis performed in a controlled environment (inert gas or vacuum), afterwhich the can is evacuated while subjected to moderate heating (e.g.,above about 200° F. (about 93° C.)) to drive off moisture and anyvolatiles, and then sealed. Thereafter, the can and its contents areconsolidated at a temperature, time, and pressure sufficient to producea consolidation having a density of at least about 99.9% of theoretical.Consolidation can be accomplished by hot isostatic pressing (HIP),extrusion, or another suitable consolidation method.

The powder consolidation is then hot worked by any of severaltechniques, such as extrusion, upset plus drawing, etc., to produce anappropriate input billet size for forging. Conditions used to producethe input billet should result in uniform ASTM 10 or finer grain sizethroughout in order to facilitate ultrasonic inspection thereof prior toforging into the final part shape.

The billet is then forged using known techniques, such as thosecurrently utilized to produce Alloy 706 and Alloy 718 rotor forgings forlarge industrial turbines but modified to take advantage of fine grainbillet techniques. Forging is performed at temperatures and loadingconditions that allow complete filling of the finish forging die cavity,avoid fracture, and produce or retain a fine uniform grain size withinthe material of not larger than ASTM 10. Notably, because chemical andmicrostructural segregation are virtually eliminated and a very finegrain size can be achieved through use of the powder metal startingmaterial, the ratio of input (billet) weight to final forging weight canbe significantly reduced. For example, it is believed the startingbillet weight can be as little as about 1.2 to about 1.5 times theweight of the finished forging, and about 1.8 to about 4 times theweight of the finish-machined rotor component. This weight reduction isenabled by the improved processibility of fine grained billet as well asthe enhanced sonic inspectibility thereof.

The resulting rotor forging preferably undergoes ultrasonic inspectingfor potential life-limiting defects. However, due to the enhancedultrasonic inspectibility of the input billet, this step of componentprocessing could potentially be eliminated which would enable morenear-net-shaped forgings to be produced and further reduce inputweights.

Inspection (if performed) is followed by finish machining by anysuitable known method to produce the finish-machined rotor component. Inorder to achieve required mechanical properties of the rotor component,prior to machining the forging is solution heat treated and aged attemperatures and times which achieve the preferred balance of propertiesfor long time industrial gas turbine service. An illustrative example ofan appropriate heat treatment process for the ARA725 alloy entails asolution heat treatment at a temperature of about 1650° F. (about 900°C.) for approximately our hours, followed by two step aging at atemperature of about 1400° F. (about 760° C.) for approximately eighthours, then cooling at a rate of 100° F. (about 56° C.) per minute toabout 1150° F. (about 620° C.) and holding for approximately eighthours, followed by air cooling.

In addition to the preferred ARA725 alloy, the process described abovecan be applied to a broad range of metal alloys whose compositions andtemperature capabilities meet a variety of specific product needs. Forexample, alloys containing conventional strengthening and/or grainboundary pinning dispersoids or nano-dispersoids such as inert oxides,nitrides, and/or carbides may be desired to impart long-term stability.Alloys containing higher levels of high temperature strengtheningelements such as cobalt, tungsten, molybdenum, tantalum, niobium, etc.,may be desired for applications requiring service up to 1800° F. (about1000° C.) or higher. In addition to direct melting and atomization ofspecific alloy compositions, mechanically alloying two or moreseparately processed powders can also be employed to obtain desiredproperties for a rotor component.

While the invention has been described in terms of a preferredembodiment, it is apparent that other forms could be adopted by oneskilled in the art. Accordingly, the scope of the invention is to belimited only by the following claims.

1. A process of producing a component from a gamma prime or gamma doubleprime precipitation-strengthened nickel-base superalloy, the processcomprising the steps of: forming a powder of the superalloy; filling acan with the powder and evacuating and sealing the can in a controlledenvironment; consolidating the can and the powder therein at atemperature, time, and pressure to produce a consolidation; hot workingthe consolidation to produce a billet of a size sufficient to form aforging of at least 2300 kg; and then forging the billet at atemperature and strain rate to produce a forging with a uniform finegrain of ASTM 10 or finer throughout.
 2. A process according to claim 1,wherein the nickel-based superalloy has a composition of, by weight,about 19 to about 23% chromium, about 7 to about 8% molybdenum, about 3to about 4% niobium, about 4 to about 6% iron, about 0.3 to about 0.6%aluminum, about 1 to about 1.8% titanium, about 0.002 to about 0.004%boron, about 0.35% maximum manganese, about 0.2% maximum silicon, about0.03% maximum carbon, the balance nickel and incidental impurities.
 3. Aprocess according to claim 1, wherein the forming step comprisesproducing a melt of the nickel-based superalloy in a controlledenvironment and then rapidly cooling the melt to produce the powder. 4.A process according to claim 3, wherein the forming step furthercomprises sieving the powder in a controlled environment to remove allparticles larger than 0.100 mm in diameter.
 5. A process according toclaim 3, wherein the forming step further comprises blending the powderwith a second powder of the nickel-based superalloy.
 6. A processaccording to claim 1, wherein the consolidation formed by theconsolidation step has a density of at least 99.9% of theoretical.
 7. Aprocess according to claim 1, wherein the forging produced by theforging step weighs at least 2300 kg.
 8. A process according to claim 1,wherein the billet formed by the hot working step weighs about 1.2 toabout 1.5 times the weight of the forging.
 9. A process according toclaim 1, wherein the billet formed by the hot working step weighs about1.8 to about 4 times the weight of the rotor component.
 10. A processaccording to claim 1, wherein the component is a rotor component of agas turbine engine.
 11. A process according to claim 1, wherein therotor component is chosen from the group consisting of turbine wheelsand spacers.
 12. A process of producing a gas turbine engine rotorcomponent from a gamma prime or gamma double primeprecipitation-strengthened nickel-base nickel-based superalloy, theprocess comprising the steps of: melting the nickel-based superalloy ina controlled environment to obtain a melt of the nickel-basedsuperalloy; convert the melt into a powder of generally sphericalparticles that are predominantly about 0.100 mm in diameter or smaller;sieving the powder in a controlled environment to remove all particleslarger than 0.100 mm in diameter; filling a mild steel can with thesieved powder and evacuating and sealing the can in a controlledenvironment; consolidating the can and the powder therein at atemperature, time, and pressure to produce a consolidation having adensity of at least 99.9 percent of theoretical; hot working theconsolidation to produce a billet of a size sufficient to form a forgingof at least 2300 kg with a uniform grain size of ASTM 10 or finerthroughout the billet; forging the billet into a forging at atemperature and strain rate to achieve a uniform fine grain of ASTM 10or finer throughout the forging; performing a heat treatment on theforging to achieve a desired balance of mechanical properties andmaintain a uniform grain size throughout of ASTM 10 or finer; andmachining the forging to produce the gas turbine engine rotor component.13. A process according to claim 12, wherein the nickel-based superalloyhas a composition of, by weight, about 19 to about 23% chromium, about 7to about 8% molybdenum, about 3 to about 4% niobium, about 4 to about 6%iron, about 0.3 to about 0.6% aluminum, about 1 to about 1.8% titanium,about 0.002 to about 0.004% boron, about 0.35% maximum manganese, about0.2% maximum silicon, about 0.03% maximum carbon, the balance nickel andincidental impurities.
 14. A process according to claim 12, furthercomprising the step of blending the powder with a second powder of thenickel-based superalloy before the filling step.
 15. A process accordingto claim 12, wherein the forging produced by the forging step weighs atleast 2300 kg.
 16. A process according to claim 12, wherein the billetformed by the hot working step weighs about 1.2 to about 1.5 times theweight of the forging.
 17. A process according to claim 12, wherein thebillet formed by the consolidation step weighs about 1.8 to about 4times the weight of the rotor component.
 18. A process according toclaim 12, wherein the rotor component is chosen from the groupconsisting of turbine wheels and spacers.
 19. A process according toclaim 18, wherein the rotor component is a component of a land-based gasturbine engine.